Add persistent sketch-constraint system (ocs_gcs port) and named-parameters handoff docs

Ports FreeCAD's planegcs 2D geometric constraint solver to Rust as a new
crates/ocs_gcs workspace member, and wires it into the document/scene model
as a persistent SketchConstraintSet: Coincident, Horizontal/Vertical,
Parallel/Perpendicular, Equal, Tangent, Distance, Angle and Radius
constraints that re-solve live (including mid-drag, not just on release),
render glyphs and a DOF badge, flag conflicts with a one-click resolver
pill, and round-trip through save/load via an XRecord blob.

Also includes docs/command_reference.md (a statically generated reference
of every registered command and its prompt sequence) and
docs/named_parameters_design.md + docs/backlog.md, which hand off the next
queued feature (a named-parameter/expression table) and consolidate every
deliberate scope-down and open gap found along the way, so a fresh session
has a complete starting point.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ui/statusbar/mod.rs

@@ -125,6 +125,16 @@ impl StatusBar {
         // Which pills the user has chosen to show on the bar.
         config: &'a StatusBarConfig,
         menu_data: StatusMenuData<'a>,
+        // Remaining degrees of freedom for the current sketch scope's
+        // persistent constraints (design doc §6.3) — `None` when the scope
+        // has no `SketchConstraintSet` yet (nothing constrained), so the
+        // badge stays invisible until it's actually relevant.
+        sketch_dof: Option<usize>,
+        // Number of redundant/conflicting constraints `solve_scope` flagged
+        // in the current sketch scope (design doc §6.4, stage 11) — 0 hides
+        // the pill entirely, so an ordinarily/fully-constrained drawing sees
+        // no new clutter.
+        sketch_conflicts: usize,
     ) -> Element<'a, Message> {
         let StatusMenuData {
             layout_names,
@@ -243,6 +253,44 @@ impl StatusBar {
                 .into(),
             );
         }
+        // Not a `StatusPill` (so not user-hideable yet): invisible until the
+        // current scope actually has a SketchConstraintSet, so a drawing
+        // that never uses parametric constraints sees no new clutter.
+        if let Some(dof) = sketch_dof {
+            let pill = if dof == 0 {
+                success_pill(crate::tf!("DOF: {dof}").into_owned())
+            } else {
+                status_pill(crate::tf!("DOF: {dof}").into_owned())
+            };
+            pills.push(
+                tip(
+                    pill,
+                    if dof == 0 {
+                        t!("Fully constrained — no remaining degrees of freedom")
+                    } else {
+                        t!("Remaining degrees of freedom in the current sketch's persistent constraints")
+                    },
+                )
+                .into(),
+            );
+        }
+        // Design doc §6.4 (stage 11): a bounded v1 of the guided conflict
+        // resolver — click removes one flagged constraint at a time. See
+        // `OpenCADStudio::resolve_one_sketch_conflict`'s doc comment for the
+        // full scope-down rationale versus the design doc's named-candidate,
+        // cyclable-preview panel.
+        if sketch_conflicts > 0 {
+            pills.push(
+                tip(
+                    action_pill(
+                        crate::tf!("⚠ {sketch_conflicts} conflicting").into_owned(),
+                        Message::ResolveOneSketchConflict,
+                    ),
+                    t!("One or more constraints in this sketch conflict or are redundant\nClick to remove one and re-solve"),
+                )
+                .into(),
+            );
+        }
         if vis(StatusPill::Ortho) {
             pills.push(
                 tip(
@@ -1034,6 +1082,25 @@ fn status_pill(label: impl Into<String>) -> Element<'static, Message> {
     .into()
 }
 
+/// Like [`status_pill`], but with the theme's `success` accent — design doc
+/// §6.3's "simple color-coded constraint state" cue: used for the DOF badge
+/// when a sketch scope has reached zero remaining degrees of freedom, so
+/// "fully constrained" is visible at a glance without opening anything.
+fn success_pill(label: impl Into<String>) -> Element<'static, Message> {
+    container(text(label.into()).size(12))
+        .style(|theme: &Theme| {
+            let palette = theme.palette();
+            container::Style {
+                background: Some(Background::Color(palette.success.weak.color)),
+                text_color: Some(palette.success.weak.text),
+                border: Border { color: palette.success.base.color, width: 1.0, radius: 4.0.into() },
+                ..Default::default()
+            }
+        })
+        .padding([4, 8])
+        .into()
+}
+
 // ── Scale popup button ────────────────────────────────────────────────────
 
 /// Shared visual root for labelled status-bar menus (units, scale, …).